In recent years, systems for distributing content of a digital copyrighted work, such as music, images and games, have been developed and some of the systems have entered a practical phase. Further, in distributing such content, a method of controlling use of content, which restricts an available period and number of times the distributed content can be reproduced and the like, has also been under consideration in view of copyright protection.
Conventionally, a server and plural terminals in a domestic network system (a home network) have been modeled such that the server distributes information including a usage rule and a content key which are necessary for using content (hereinafter the information referred to as “license”) and that the terminals use the content to, for example, reproduce the content, by using the license distributed from the server.
Further, the license distributed from the server is held by each of the terminals. The terminal uses the content by using the license that the terminal holds. When accumulating a license, the terminal encrypts the license by using a unique key which is unique to the terminal, and accumulates the encrypted license. By doing this, even when the accumulated license is duplicated in other terminals, the duplicated license can not be decrypted. Accordingly, it is not possible to use the content by using the duplicated license. This enables secure copyright protection. Further, when transferring such license between plural terminals, the license is usually allowed to be transferred only within a certain area, and a transfer in an unrestricted area is not allowed in view of protection of content right and privacy protection for an owner of the terminal. It is usually considered that a transfer is allowed only between terminals owned by the same user.
As a conventional technique for achieving this, it has been considered that a single domain is set for plural terminals owned by the same user, and a domain key is generated for each domain and distributed to each of the terminals. When outputting the license outside, the terminal decrypts a code of the accumulated license which has been encrypted with the unique key. Then, the terminal encrypts it with the domain key and output it. Consequently, this license can be decrypted at terminals belonging to the same domain, but can not be decrypted at terminals not belonging to the same domain. As described above, it is achieved that, when two terminals belong to the same domain, the license is allowed to be transferred between the two terminals, but otherwise not allowed to be transferred (see Patent Reference 1, for example).
According to the conventional technique described above, when transferring a license between terminals, a source terminal encrypts the license with the domain key and then output it, and a destination terminal receives the license which is encrypted with the domain key as an input, decrypts it with the domain key and then use it. Here, in the case where the license is encrypted with the domain key at the source terminal, the license before the encryption is automatically deleted within the terminal at the time of the encryption.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2000-181803